1. Field of the Invention
The present invention relates to an image forming apparatus that forms an electrostatic latent image by exposing a surface of a photoconductor with a light beam, visualizes the electrostatic latent image by using toner, and fixes the electrostatic latent image on recording paper by the toner, and a method for controlling an image density of the image forming apparatus. Particularly, the present invention relates to an image forming apparatus that stabilizes the amount of adhering toner or densities of a high-density image.
2. Description of Related Art
In image forming apparatuses, the amount of toner adhering to an image varies depending on the individual apparatuses, a change with time or fatigue of a photoconductor and the toner, a change in temperature and/or humidity around the apparatus, or others. Taking this into account, conventional techniques for obtaining optimal values of a charging bias and a developing bias to stabilize image densities have been proposed (for example, Japanese Unexamined Patent Publication No. Hei 10-239924).
Japanese Unexamined Patent Publication No. 2002-214852 discloses a technique for stabilizing image densities of toner images in a wide density range. According to this technique, solid patch images are formed under various conditions for preparing patches, optical densities of the respective patch images are measured by a patch sensor, and higher-density side correlation data matching to a higher-density side target density and lower-density side correlation data matching to a lower-density side target density are collected from measurement data. Then one piece of correlation data is obtained that belongs to a product set of the higher-density side correlation data and the lower-density side correlation data. From the correlation data, an optimal exposure energy and an optimal developing bias are determined.
Japanese Unexamined Patent Publication No. 2006-003816 discloses a technique for solving a problem that appropriate density compensation data cannot be obtained due to variations of dots and densities on image data of a test pattern, the variations caused by a variation based on gradation expressions of the test pattern. With this technique, a reference test pattern which is expressed by the gradation expressions of a dot array and/or a dot size expressing a single pixel is formed on an image carrier, the image density of the reference test pattern is detected, and the density compensation data is prepared on the basis of the detected density.
Both techniques in the above publications, Japanese Unexamined Patent Publication Nos. Hei 10-239924 and 2002-214852, are to form black solid patches on a photoconductor and to detect densities of the black solid patches. In Japanese Unexamined Patent Publication No. 2006-003816, a plurality of rectangular images of the same density are arranged to form a gradation image and respective densities of the gradation images are gradually changed to form an image as a density pattern.
When black solid patches are formed, however, the black solid patches absorb light and cause a decrease in sensitivity of a sensor. When it comes to black solid patches of moderate-density, which are formed by modulating the pulse width of light exposure and the density of the solid patches of the moderate-density is detected by a density sensor, the density of the black solid patches is changed by stains on the sensor or the long-term-use of the sensor such as the sensor's life span. Also, the black solid patches consume a larger amount of toners and they are problematic. Regarding Japanese Unexamined Patent Publication No. 2006-003816, another problem has been identified that kinds of the reference test pattern are limited.